1. Field of the Invention
The invention relates generally to an electrode for implantable stimulation devices such as heart pacemakers or defibrillators. The invention relates further to implantable leads and stimulation devices such as heart pacemakers or defibrillators which employ such an electrode. Moreover, the invention relates to the use of the electrode for diagnosing the condition of stimulated tissue. In particular, the invention relates to an electrode which is adapted to electrically and mechanically transfer stimulation energy to tissue, to an electrode which is adapted to electrically and mechanically receive electrical and mechanical evoked response of the tissue to which stimulation energy has been transferred and to an electrode which is adapted for a combination of the two.
2. Description of the Prior Art
The life span of most pacemakers is dictated by the rate at which their batteries drain. Thus, a substantial effort has been directed toward minimizing the amount of energy used by pacemakers, while ensuring that the devices continue to deliver effective therapy. For example, demand pacemakers effectively reduce the battery drain by delivering pacing pulses only when required, i.e. if the pacemaker has not detected any spontaneous activity. Another way to reduce the current consumption is to minimize the amplitude and/or the duration of the stimulation pulse to a value just above the threshold. However, there are for example times when the heart emits an electrical signal, without providing a corresponding mechanical contraction (electromechanical dissociation). However, the pacemaker detects and interprets the electrical signal as an intrinsic beat or an evoked response. There are also times when the heart does not respond normally with increased cardiac output for increased stimulation rate as for example for patients with coronary artery disease during angina pectoris.
A way of minimizing the amount of energy needed for defibrillation, while ensuring that the defibrillators continue to deliver effective therapy, is disclosed in U.S. Pat. No. 5,433,731 a defibrillator having means for supplying the heart with a mechanical shock instead of an electrical shock. One embodiment discloses an electrode for supplying a defibrillation pulse, whereby the electrode is provided with an element on its distal exterior, which presses against the heart tissue and converts the electrical energy into mechanical energy. The element can for example be a piezoelectric element.
U.S. Pat. No. 5,304,208 discloses a cardiostimulator device having an electrode including an acceleration sensor for detecting the acceleration to which the cardiac mass is subjected as a reaction to any contraction whatsoever of the cardiac mass. The acceleration sensor is solely sensitive to inertial forces and can therefore be located in an entirely rigid capsule and consequently be entirely insensitive to the pressure in the ventricle or the atrium, and to pressure which the cardiac wall can exert, particularly on the distal electrode.
It is an object of the present invention to provide an implantable electrode, an electrode lead embodying an implantable electrode, and an implantable stimulation device employing an electrode lead, which avoid the aforementioned disadvantages of known leads and stimulators.
This object is inventively achieved in accordance with the principles of the present invention in an implantable electrode for an implantable stimulation device, the implantable electrode having a conductive core covered by a biocompatible piezoelectric material adapted to be in direct contact with tissue when implanted, and wherein the entire piezoelectric surface of the implantable electrode is adapted for electrically and mechanically transferring stimulation energy to the surrounding tissue, and/or is adapted to electrically and mechanically receive electrical and mechanical evoked or intrinsic responses of tissue, to which stimulation energy can be transferred.
The above object is also achieved in an electrode lead having an implantable electrode as described above, and further having a conductor for delivering stimulation energy to the piezoelectric material and/or for conducting signals from the piezoelectric material, the signals from the piezoelectric material representing the aforementioned electrical and mechanical evoked or intrinsic response of the tissue.
The above object is also achieved in an implantable stimulation device for stimulating tissue having an electrode lead with an implantable electrode as described above, and further having a stimulation pulse generator for delivering stimulation energy to the implantable electrode, and a detector for receiving signals from the implantable electrode corresponding to the aforementioned electrical and mechanical evoked or intrinsic response of the tissue.
An advantage of the invention is that it is possible to more reliably stimulate heart tissue and to detect a heart contraction. As a result a lower energy consumption is ensured.
According to the invention, the piezoelectric electrode is formed of a biocompatible piezoelectric material adapted to be in direct contact with the tissue, it surprisingly has been found that the conductive layer hitherto believed necessary can be excluded. In one embodiment the piezoelectric electrode is the tip electrode and in another embodiment the piezoelectric electrode is the ring electrode. In a preferred embodiment the stimulation pulse generator supplies the electrode with a chopped stimulation pulse.